Facebook added status details to its Mobile Messenger application. You can see if someone has read a message you've sent, if they're typing a reply, and what general region they are physically replying from (that one could be problematical). Apple iMessage and BlackBerry Messenger users are already accustomed to these features.
Twitter added a mouse-hover behavior to tweets on the Twitter.com site. Now when you hover over a tweet, interaction choices (Reply, Retweet, and Favorite) appear. They're not there otherwise, unless you click on the persistent "Expand" option that's on each tweet. More